Apparatus for cleaning surfaces

ABSTRACT

The apparatus for cleaning surfaces comprises a body ( 1 ) comprising a tank ( 2 ) for containing an abrasive material and a first duct ( 19 ) for a flow of compressed air which extends adjacent to said tank ( 2 ) along the whole length of said tank ( 2 ). The apparatus comprises as well a mixing valve ( 31 ), associated with said tank ( 2 ), for mixing said abrasive material with said compressed air.

TECHNICAL FIELD

The present invention concerns an apparatus for cleaning metallic and non-metallic surfaces.

BACKGROUND ART

It has been known the use of apparatuses for cleaning surfaces through a process known as sandblasting.

The sandblasting consists in propelling a jet of compressed air and abrasive material on the surface to be treated to remove the superficial layer of material. Alternatively, sandblasting can be used for creating inscriptions or images on marble, stone, wood or glass.

Traditional cleaning apparatuses comprise a tank for abrasive material connected, through a mixing valve, to a mixing chamber in which a predetermined flow of compressed air, generally produced by a compressor, is conveyed. A sprayer gun provided with commands for spraying, through a suitable nozzle, a jet of air and abrasive material is connected with the mixing chamber through a suitable duct.

Patent US 2014/0065933 discloses a type of cleaning apparatus comprising a hopper, associated at its top with a framework, predisposed to contain a sandy material. In a lower part of the hopper, a mixing chamber is placed, being predisposed to receive the sandy material and air. In the chamber a vertical duct is inserted for the access of the air, connected with an air regulation system. The chamber is connected, through an outlet duct, with a sprayer gun so as to allow the passage of the flow of air and sand. A duct for the water supplied by a suitable pump is connected as well with the gun in order to allow spraying of a flow of air, sand and water through a nozzle placed at one end of the sprayer gun.

Patent WO 2008/035236 discloses a further apparatus for cleaning surfaces comprising a tank for containment of an abrasive material and a duct for the flow of compressed air, which extends adjacent to the tank. A mixing valve is associated with the tank for the mixing of the abrasive material with the compressed air. Such mixing valve is connected with a sprayer gun of a flow of compressed air and abrasive material through connection means.

Cleaning apparatuses generally use silica abrasives. Alternatively, carbonates are used, for example baking soda, which is preferable from an ecologic point of view and particularly efficient as it leaves unaltered the support, even in case of polished surfaces.

Furthermore, cleaning apparatuses are predisposed for working with baking soda either accompanied by mineral materials or by vegetal materials, since these materials, having very different physical, mechanical and granulometric characteristics, require different types of apparatuses.

This is to the detriment of the practicality of use and entails high costs for the management of a plurality of different apparatuses.

A further problem complained in the field is the fact that the abrasive material, in particular the material consisting of grains of small size, tends to remain adherent to the walls of the tank, especially at the perimetral edge of a lower base of the tank, with consequent waste of material and discontinuity in the flow of abrasive material released from the tank.

Occlusion phenomena have also been observed at the mixing valve, compromising the continued spraying and increasing working time.

DISCLOSURE

The task of the present invention is that of solving the cited problems, devising an apparatus for cleaning surfaces which allows the continued and constant spraying of material from the sprayer guns, whenever it is required, as well as the correct storage, inside the suitable tank, of the chosen material.

A further scope of the present invention is that of providing a portable apparatus for cleaning surfaces which eases the transport and the manoeuvrability of the same apparatus.

A further scope of the present invention is that of providing an apparatus of simple constructive and functional conception, provided with a surely reliable functioning, versatile use as well as relatively economic cost.

The cited scopes are attained, according to the present invention, by the apparatus for cleaning surfaces according to claim 1.

The apparatus for cleaning surfaces comprises a mixing valve for mixing an abrasive material with a flow of compressed air.

The mixing valve comprises a first inlet duct for the passage of the flow of compressed air and a central duct which extends along a central axis and defines a mixing chamber.

The central duct is connected with the first inlet duct.

The mixing valve comprises a second inlet duct for the passage of the abrasive material, communicating with the central duct. The second inlet duct extends along a longitudinal axis inclined with respect to the central axis of the central duct according to an acute angle.

Preferably, such acute angle is comprised in a range between 30° and 45°.

The mixing valve is associated with a tank for containing the abrasive material.

The apparatus comprises a body which comprises the above mentioned tank and a first duct for a flow of compressed air which extends adjacent to the tank, along the whole length of the tank. The duct is predisposed for allowing the flow of compressed air both directly inside the tank, for allowing an efficient storage of the abrasive material inside the tank, and towards the mixing valve.

The apparatus comprises a conveying member mounted inside the tank, in a lower position, having a hollow conical shape, asymmetric with respect to the revolution axis.

Preferably, the conveying member shapes at its top a collecting opening for the abrasive material contained in the tank and an opposite eccentric opening toward which said abrasive material is conveyed.

Alternatively, the apparatus comprises a second duct which extends inside said tank along a longitudinal direction, substantially parallel to said first duct, predisposed to convey compressed air towards said second inlet duct of the mixing valve so as to ease the passage of said abrasive material towards said mixing valve and reducing the occlusion phenomena.

Preferably, the second duct comprises an inlet portion which extends superiorly with respect to said abrasive material contained in said tank so as to allow the inlet of a part of compressed aid inside said second duct and an outlet portion which is inserted in a discharge opening which is connected with said second inlet duct of the mixing valve to convey the compressed air to the second inlet duct.

Sprayer means are connected with the mixing valve.

Advantageously, the sprayer means have at one end a spraying nozzle, predisposed to spray the flow of compressed aid mixed with the abrasive material on a surface to be treated.

Preferably, the spraying nozzle comprises a hollow cylindrical body, having an inlet opening and an opposite outlet opening.

Preferably, the spraying nozzle comprises a portion of narrowed transversal section, starting from which a guide channel is shaped, having a shape diverging towards the end for the exit of the flow of compressed air and abrasive material.

Advantageously, the guide channel diverges of an acute angle with respect to a longitudinal axis of the spraying nozzle so as to spray a jet of substantially conical shape having a homogeneous distribution of the abrasive material in the volume of the same jet.

Preferably, the acute angle is comprised in a range between 2° and 8°.

Alternatively, the guide channel has a substantially cylindrical shape and such configuration allows to obtain a jet of outgoing air comprising particles of abrasive material having a higher propagation speed.

Advantageously, a suitable shutter member is arranged coaxially to the second inlet duct of the mixing valve. The shutter is associated with a regulation valve, which allows to adjust, by operating the same valve, the transversal section of the second inlet duct and, thereby, the quantity of abrasive material to be mixed with the flow of compressed air.

DESCRIPTION OF DRAWINGS

Details of the invention shall be more apparent from the detailed description of a preferred embodiment of the apparatus for cleaning surfaces according to the invention, illustrated for indicative purposes in the attached drawings, wherein:

FIG. 1 shows a perspective view of the apparatus;

FIG. 2 shows a vertical cross-section view of the apparatus according to the invention;

FIG. 3 shows a down-up view of the apparatus;

FIGS. 4 and 5 show a view of the apparatus respectively according to the tracing planes IV-IV and V-V of FIG. 1;

FIG. 6 shows a longitudinal cross-section view of the apparatus according to an embodiment;

FIGS. 7, 8, 9 show respectively a perspective, lateral and top-down view of a detail of the apparatus shown in FIG. 6;

FIG. 10 shows a longitudinal cross-section view of the apparatus according to a different embodiment in a load step of the apparatus;

FIG. 11 shows a perspective exploded view of a detail of the apparatus shown in FIG. 10;

FIG. 12 shows a perspective exploded view of a mixing valve of the apparatus according to the invention;

FIG. 13 shows a perspective exploded view of a different embodiment of the mixing valve;

FIG. 14 shows a lateral view of a magnified detail of the mixing valve shown in FIG. 12, indicated with D;

FIG. 15 shows a cross-section view of the detail shown in FIG. 14;

FIG. 16 shows a front view of a detail of the apparatus;

FIG. 17 shows a view of the above mentioned detail of the apparatus according to the tracing plane XVII-XVII;

FIG. 18 shows a longitudinal cross-section view of a further detail of the apparatus;

FIG. 19 shows a longitudinal cross-section view of a different embodiment of the above mentioned detail.

BEST MODE

With particular reference to such figures, the apparatus for cleaning surfaces according to the present invention has been indicated in its entirety with 1.

The apparatus 1 comprises a tank 2 predisposed to contain an abrasive material, closed at its top by a lid 3 provided with a refill opening 4 for the replenishment and the further refills of material.

Under the lid 3, a spring door 3 is placed so as to hermetically close the refill opening 4 and allowing the opening of the lid 3 exclusively towards the inside of the tank 2.

The used abrasive material is different depending on the type of working to be carried out. Such material can be, for example, of the mineral or vegetal or iron metal type. As an alternative, carbonates can be used such as baking soda. The above mentioned materials con be used singularly or, alternatively, combined one another thus creating a blend. The size of the grains of the above mentioned materials can be bigger than 0.06 mm, preferably in the range 0.06-1.50 mm.

The tank 2 comprises a hollow cylindrical body preferably obtained through extrusion of aluminium, for making the apparatus lighter and for easing the transport, making it possible also by shoulder.

The tank 2 is associated at the top with the above mentioned lid 3 and at the bottom with a base 6 through assembling means 7 comprising a series of tie-rods which extend longitudinally, along the whole length of the cylindrical body, so as to shape a steady and durable tank 2, able to support the high pressure attained inside the same tank.

The tie-rods 7 are fixed to the lid 3, at one end, through hook means 8 of the screw type.

At the opposite end the tie-rods have portions 7 a suitably manufactured, for example threaded, predisposed to be inserted in corresponding seats 9 of conjugated shape obtained on the base 6.

The base 6 has an emptying opening 10 closed by a suitable emptying valve 11.

The base 6 comprises as well a discharge opening 12 for the abrasive material and a further discharge opening 13 for the compressed air.

A couple of feet 14 for supporting the apparatus 1 is frontally associated with the base 6.

At a rear part of the base 6, a couple of wheels 15 is mounted so as to ease the transport of the apparatus.

The lid 3 and the base 6 are, for example, made up of a plate preferably made of aluminium, which gives further lightness.

The tank 2 shapes an upper portion, predisposed to contain compressed air, which constitutes a compression chamber so as to exert a constant pressure on the introduced material and, hence, carry out the correct storage of the same material inside the tank 2.

Inside the tank 2, inferiorly, a conveying member 16 is mounted, having a hollow conical shape asymmetric with respect to the revolution axis.

The conveying member 16 shapes at the top a receiving opening 17, which coincides with the base of the cone, through which the material contained inside the tank 2 is collected and suitably guided towards an eccentric opening 18 opposite to the receiving opening 17 (see FIGS. 6-9). The eccentric opening 18 is placed above the discharge opening 12.

Adjacent to the tank 2, a first duct 19 extends for the whole length of the cylindrical body of the tank 2, the first duct 19 communicating with the compression chamber, and being predisposed to allow the passage of compressed air.

According to an embodiment, inside the tank 2 a second duct 20 is arranged, extending along a longitudinal direction, substantially parallel to the first duct 19, predisposed to convey compressed air towards the discharge opening 12.

More in detail, the second duct 20 is provided with an inlet portion 200 protruding with respect to the abrasive material, that is, which extends superiorly with respect with the abrasive material contained in the tank 2 so as to allow the access of a part of the compressed air introduced in the chamber inside the second same duct 20.

Such duct 20 has as well an outlet portion 201, which is inserted in the discharge opening 12 of abrasive material, to convey the compressed air in an inlet duct of the abrasive material, towards a mixing chamber, as it is better explained in the following, so as to ease the passage of the material in the above mentioned chamber and reduce the occlusion phenomena (see FIG. 10).

The first duct 19 comprises, at the top, a first transversal hole 21 c and a second transversal hole 22. The first hole 21, obtained at the above mentioned compression chamber, connects the outside with the inside of the tank 2 (see FIG. 4) while the second hole 22 connects the outside with the sole first duct 19 (see FIG. 5).

In the aforementioned holes 21, 22, a valve 23 for the entry of pressurized air both in the duct 19 and inside the tank 2, and a safety valve 24 for the exit of air, in case of overpressure in the tank 2, are respectively applied.

It is possible to provide that the first duct 19 is provided with a third hole 25, arranged below the second hole 22, which puts in connection the duct 19 with the outside. At the third hole 25, a discharge valve 26 is provided, predisposed to allow the discharging of air under pressure remaining in the tank 2 whenever stopping the spraying, as it is better explained in the following (see FIG. 1). In this way, the working time is reduced as the exceeding air is quickly expelled from the tank 2, and then the pressure value of the air in the chamber in rest condition of the apparatus 1 is promptly restored.

The pressurized air is supplied by a compressor, not shown in the figures, through a supply duct 27 communicating at a terminal part with an anti-condensation filter 28.

The anti-condensation filter 28 is placed below a flow control valve 29 of the compressed air, for example of pneumatic type, which is connected with a connection duct 30. The connection duct 30 is associated at an opposite end with a pressure regulator 31, connected with the valve 23 for the entry of pressurized air.

The pressurized air can reach pressure values higher than 0.2 bar, preferably in the range between 0.2 bar and 10 bar.

A mixing valve 31 is associated with the tank 2 and is predisposed to receive the abrasive material and, in case the second duct 20 is provided, also pressurized air, through the discharge opening 12 and the sole flow of compressed air through the further discharge opening 13 obtained at a terminal part of the duct 19. In particular, the mixing valve 31 is placed below the base 6.

The mixing valve 31 comprises a body 32 internally shaping a first inlet duct 33 for the passage of the flow of compressed air communicating, by means of a central duct 34, with a second inlet duct 35 for the abrasive material or of the abrasive material with compressed air.

The central duct 34 extends along a central axis B in a direction transversal, preferably orthogonal, to the first inlet duct 33 and defines a mixing chamber for the compressed air and the abrasive material.

At one end, the central duct 34 shapes a connection portion 36 predisposed to allow the coupling of the mixing valve 31 with a join member 37 for a flexible rubber pipe 38.

The connection portion 36, internally, has a threading so that a corresponding portion of conjugated shape 37 a of the join member 37 can be inserted by screwing (see FIG. 12, 14 e 15).

Alternatively, the join member 37 is fixed to the valve 31 through suitable join means 39.

In particular, the connection portion 36 has a threading made on an internal surface thereof, so that it can be coupled, by screwing, to the above-mentioned join means 39 (see FIG. 13).

The join means 39 comprise a ferrule, provided with an inner threading having shape conjugated with the shape of the connection portion 36, arranged coaxial to the join member 37 and, in an assembled configuration, in abutment with the body 32 of the mixing valve 31. Between the ferrule 39 and the join member 37 a seal 40 is interposed, for example of the O-ring type.

The join member 37 is made of a material having high hardness and resistance to abrasion, for example steel or tungsten carbide.

The second inlet duct 35 extends from the discharge opening 12 to the central duct 34 along a longitudinal axis A inclined by an acute angle α with respect to the central axis B of the central duct 34.

The acute angle α is preferably comprised in the range between 30° and 45° (see FIG. 17), for which optimal results are obtained for the efflux regularity.

At the second inlet duct 35, a suitable shutter member 41 is placed, which allows to adjust, upon activation of a regulation valve 42 associated therewith, the cross-section of the same second inlet duct 35 and, hence, the amount of abrasive material to be mixed with the compressed air flow.

Preferably, the shutter member 41 is made of steel, to confer optimal abrasion resistance.

The flexible tube 38 is associated at one end with the join member 37 and at the opposite end to suitable sprayer means 43 constituted for example by a sprayer gun 44.

The sprayer gun 44 comprises a series of elements, not illustrated for simplicity, suitably assembled so as to give the sprayer gun 44 itself an ergonomic configuration. These elements are preferably obtained by extrusion of aluminium.

The sprayer gun 44 has at one end a spraying nozzle 45, through which a jet of air and abrasive material is sprayed on the surface to be treated.

The spraying nozzle 45 comprises a hollow cylindrical body 46 having an inlet opening 47 and an opposed outlet opening 48 for the flow of air and abrasive material.

Preferably, the spraying nozzle 45 is made of a material with high hardness and abrasion resistance, preferably tungsten carbide.

The spraying nozzle 45 comprises a cross-section narrowing portion 49, starting from which the thickness of the cylindrical body 46 progressively reduces until the exit end 48, thus shaping a guide channel 50 for the flow.

More in particular, the guide channel 50 diverges with an acute angle β with respect to a longitudinal axis C of the spraying nozzle 45 so as to shape a channel of substantially conical shape, diverging towards the exit, for a jet of compressed air and abrasive material. Thanks to this conformation, the distribution of the outgoing material is particularly homogeneous, giving rise to a jet of substantially conical shape, with a homogeneous distribution of the material in the volume occupied by the jet itself (see FIG. 18).

The diverging angle β is preferably comprised in a range between 2° and 8°, in which the homogeneity of the distribution is optimal.

Starting from the narrowing portion 49, the nozzle 45 shapes, towards the entrance, at the side opposite to the outlet channel 50, a converging inlet channel 51, so as to determine, in combination with the above-mentioned portions of the nozzle 45, a Venturi effect.

In practice, the thickness of the cylindrical body 46 gradually decreases towards the inlet opening 47 thus shaping the convergent inlet channel 51, for the entrance of the flow of compressed air and abrasive material.

According to a different embodiment, the thickness of the cylindrical body 46, starting from the above mentioned narrowing portion 49 towards the outlet opening 48, remains unchanged, therefore the shaped guide channel 50 is substantially cylindrical (see FIG. 19). In such case, the configuration of the guide channel 50 allows to obtain an outgoing air jet comprising particles of abrasive material provided with a higher propagation speed. Such kind of flow is suitable in case a particularly aggressive treatment is required for the surface.

The sprayer gun 44 is provided as well with suitable command means 52, which can be operated by the user to operate or deactivate the spraying of compressed air and abrasive material through the spraying nozzle 45.

In particular, these command means 52 are associated with the valve 29 of regulation of the flow through connection means 53, for example of the type of connection pipes, to send control signals to the valve 29 to set it in a closure or opening configuration.

Preferably, the command means 52 comprise a lever member operable by hand.

The functioning of the apparatus for cleaning surfaces is easy to understand from the preceding description.

When starting the compressor, a jet of compressed air flows from the supply duct 27 through the anti-condensation filter 28 and, in succession, to the regulation valve 29 of the compressed air flow and to the pressure regulation device 31, until the inlet valve 23, from which the flow is distributed in the first duct 19 and in the tank 2. In particular, the compressed air occupies the upper portion of the tank so as to exert a constant pressure on the material. In case a second duct 20 is present, the compressed air is introduced as well in the second duct 20 and eases the flowing of the abrasive material from the tank 2 to the second inlet duct 35 until the central duct 34 of the mixing valve 31 where it is suitably mixed with the flow of compressed air introduced through the first duct 19.

The flow of compressed air and abrasive material mixed in the mixing valve 31 is conveyed through the flexible pipe 38 to a sprayer gun 44 which, when activated by the operator, sprays the jet on the surface to be treated.

When the user wishes to stop the outgoing jet of compressed air and abrasive material, he operates the command means 52 of the sprayer gun 44. The command means 52 send a command signal to the valve 29 of regulation of the flow so as to set it in a closure configuration and thus stop the passage of compressed aid from the compressor to the inlet valve 23.

The exceeding air remaining in the tank 2 at the end of the spraying is automatically expelled through the discharge valve 26 allowing a quick restoration of the air pressure value, in rest conditions, in the chamber of the tank 2 and then making the apparatus ready to use in a quick time.

The apparatus for cleaning surfaces therefore attains the scope of allowing the continued and constant emission of the material from the sprayer gun whenever it is necessary.

It has been observed, in fact, that the inclination of the second inlet duct of the mixing valve with respect to the central duct of an angle comprised in the range between 30° and 45° allows to significantly reduce the phenomena of occlusion of abrasive material, thus allowing a constant flow of material towards the mixing chamber. Moreover, in the embodiment comprising the second duct, the feeding of the abrasive material to the mixing valve is eased by the above mentioned second duct that introduces compressed air in the feeding duct of the abrasive material, thus considerably reducing the occlusion phenomena, which are frequent mostly in case of use of abrasive material having grains of reduced size.

In this way, continuity in the flow of abrasive material and compressed air outgoing from the mixing valve is ensured, with consequent optimization of production times.

It has been observed, as well, that the inclination of the guide channel of the nozzle of the sprayer gun is optimal in terms of homogeneity of the product jet at the exit. The abrasive material is in fact uniformly distributed in the volume of the obtained conical jet, thus ensuring a uniform distribution of the abrasive material on the surface to be treated.

Another important aspect is represented by the fact that the uniform thrust exerted by the air fed into the compression chamber makes constant the outflow of material from the tank towards the mixing chamber and allows a proper storage of the material in the tank.

In this respect, a significant contribution to the regularity of the above-mentioned flow is due to the use of the conveying member, which permits to prevent problems of adhesion of the material to the tank, in particular at the peripheral edge of the base of the tank, easing the outflow of the material towards the discharge opening.

This aspect is particularly advantageous in case a material consisting of grains of reduced size is used and low pressure is used for compression on the material, as well as in case it is required to frequently replace the type of used abrasive material and then to minimize the adherence of the material to the tank to ease its replacement.

A further aspect to be underlined is that the apparatus allows to reduce working time, in particular the discharge time of the exceeding compressed air and therefore the time required between two subsequent sprayings, thanks to the discharge valve which allows to quickly discharge air remaining in the tank at the end of each spraying. Thus it is also achieved a saving of abrasive material since, in the absence of such a valve, the exceeding air is expelled, at the end of each spraying, through the nozzle of the sprayer gun together with the abrasive material. The reduction of the discharge time increases, at the same time, the safety of the user since the jet exiting from the sprayer gun is stopped in a reduced time.

It was also observed that the presence of the discharge valve considerably reduces the phenomena of turbulence that may occur at the compressed air inlet valve, which cause the accumulation of abrasive material in the feed circuit of compressed air, in particular in the pressure regulator device.

The present invention attains also the scope of providing a versatile apparatus that can be used for materials having different mechanical, physical and granulometric characteristics, thanks to the above-mentioned mixing valve, which allows to suitably dose the quantity of abrasive material to be mixed by means of the regulation valve.

The apparatus for cleaning surfaces also allows an easy transport mainly thanks to the use of materials, such as aluminium, which significantly lighten the overall weight of the apparatus and make also possible alternative mode of use thereof. In case of embodiments of the apparatus of small size, the operator can easily support the weight of the apparatus, for example by loading it on his shoulder.

The ergonomic features of the sprayer gun also gives the apparatus an easy manoeuvrability.

A further aspect to be underlined is the use of materials of high hardness and abrasion resistance, in particular tungsten carbide, for the realization of members such as the obstruction unit, the spraying nozzle and the connection member, particularly subjected to abrasion phenomena. In this way, a greater reliability and durability is given to the apparatus.

The apparatus described as an example is susceptible of numerous modifications and variants according to the different exigencies.

In practice, the embodiment of the invention, the materials used, as well as the shape and dimensions, may vary depending on the requirements.

Should the technical characteristics mentioned in each claim be followed by reference signs, such reference signs were included strictly with the aim of enhancing the understanding the claims and hence they shall not be deemed restrictive in any manner whatsoever on the scope of each element identified for exemplifying purposes by such reference signs. 

1. An apparatus for cleaning surfaces, the apparatus comprising: a body comprising a tank for containing an abrasive material and a first duct for a flow of compressed air, said first duct extending adjacent to said tank along a whole length of said tank; a mixing valve, associated with said tank for mixing said abrasive material with said compressed air; a sprayer means for connecting with said mixing valve, said first duct being predisposed for allowing said flow of said compressed air both directly into said tank to allow storage of said abrasive material inside said tank, and said flow of said compressed air towards said mixing valve, said mixing valve comprising a first inlet duct for passage of said flow of compressed air; a central duct extending along a central axis, shaping a mixing chamber and communicating with said first inlet duct, a second inlet duct for passage of said abrasive material, said second inlet duct communicating with said central duct, said second inlet duct extending along a longitudinal axis inclined with respect to said central axis of said central duct of an acute angle; a conveying member mounted inside said tank, in a lower position, having a hollow conical shape being asymmetric with respect to a revolution axis of said conveying member.
 2. An apparatus according to claim 1, wherein said conveying member defines a receiving opening in an upper part of said conveying member for said abrasive material contained in said tank and an opposite eccentric opening towards which said abrasive material is conveyed.
 3. An apparatus according to claim 1, wherein said acute angle is comprised in a range between 30° and 45°.
 4. An apparatus according to claim 1, wherein said sprayer means has a spraying nozzle at one end of said sprayer means, said nozzle being predisposed to spray said flow of compressed air mixed with said abrasive material on a surface to be treated, said nozzle comprising a hollow cylindrical body having an inlet opening and an opposite outlet opening.
 5. An apparatus according to claim 4, wherein said spraying nozzle comprises a portion having a narrowing of transversal section from which a guide channel having a shape diverging towards said outlet opening is defined for said flow of compressed air and said abrasive material.
 6. An apparatus according to claim 5, wherein said guide channel diverges of an another acute angle with respect to a longitudinal axis of said spraying nozzle so as to spray a jet having a substantially conical shape with a homogeneous distribution of said abrasive material in a volume of said jet.
 7. An apparatus according to claim 6, wherein said another acute angle is comprised in a range between 2° and 8°.
 8. An apparatus according to claim 1, further comprising: a shutter member, arranged inside said second inlet duct, associated with a regulation valve, which allows a transversal section of said second inlet duct to adjust upon activation of said regulation valve. 